Japanese Patent Application Publication No. H06-222665 discloses a developing apparatus that toner cartridge is detachably mounted on the top of the main casing. In a developing apparatus of this type, toner is supplied from the toner cartridge to a toner hopper, thence into the developing chamber that accommodates a developing roller and a supply roller. In the developing chamber, the toner is supplied onto the supply roller. As the supply roller and the developing roller rotate in contact, the toner is supplied to the developing roller. Then, the toner is supplied from the developing roller to the electrostatic latent image formed on the circumferential surface of the photosensitive drum. As a result, the electrostatic latent image is developed.